This invention relates to a method of producing an aluminum support for a printing plate, and more particularly to a method of producing a grained aluminum support for an offset printing plate.
Conventionally, an aluminum plate (including aluminum alloy) has been used as a printing plate, such as an offset printing plate. Usually, in offset printing, it is necessary to apply a suitable adhesion and a suitable amount of water between the surface of the aluminum plate and a photosensitive layer.
The surface of the aluminum plate should be uniformly and finely grained to meet the aforesaid requirements. This graining process largely affects a printing performance and a durability of the printing plate upon the printing process following manufacture of the plate. Thus, it is important for the manufacture of the plate whether such graining is satisfactory or not.
In general, an alternating current electrolytic graining method is used as the method of graining an aluminum support for a printing plate. There are a variety of suitable alternating currents, for example a sinewaveform, a squarewaveform, a special alternating waveform and the like. When the aluminum support is grained by alternating current supplied between the aluminum plate and an opposite electrode such as a graphite electrode, this graining is usually conducted only one time, as the result of which, the depth of pits formed by the graining is small over the whole surface thereof. Also, the durability of the grained printing plate during printing will deteriorate. Therefore, in order to obtain a uniformly and closely grained aluminum plate satisfying the requirement of a printing plate with deep pits as compared with their diameters, a variety of methods have been proposed as follows.
One method is a graining method to use a current of particular waveform for an electrolytic source (Japanese Patent Laid-Open No. Sho 53-67507). Another method is to control a ratio between an electricity quantity of a positive period and that of a negative period at the time of alternating electrolytic graining (Japanese Patent Laid-Open No. Sho 54-65607). Still another method is to control the waveform supplied from electrolytic source (Japanese Patent Laid-Open No. Sho 55-25381). Finally, another method is directed to a combination of current density (Japanese Patent Laid-Open No. Sho 56-29699).
Additionally, Japanese Patent Examined Publication No. Sho 61-60797 discloses obtaining a uniform graining surface as a result of supplying an alternating current to the aluminum plate, in which at least one of the positive period and negative period includes a rest period of 0 Volt, so that the electricity quantity of the positive period may be larger than that of the negative period.
However, in the case of using an alloy containing a large variety of ingredients for the aluminum plate, such as JIS3003 material, an irregular yield of a small amount of ingredients among aluminum lots is likely to transform the graining form and to change the printing performance.
Although the methods disclosed in the above publications satisfy the requirements for uniform pits, more satisfactory printing performance and more uniform pits have recently been required.